Topic: cb750 cam chain roller problem ???

[paulages]

i picked up a '75 cb750K last night for free, due to a "siezed engine." the guy said it ran when he bought it, but then one day he went to kick it over and it just went "clunk" and stopped. after i got it on my stand, i found that the engine has a range of rotation, but something is physically blocking it from rotating fully. anyway, i pulled the engine and found that the cam chain roller is doing something strange.

when rotated, the top roller moves forward (increasing tension), then at a certain point snaps back again. a few degrees after this it locks up. upon examining the tensioner, i found that it was all the way out and locked down. at first i thought the lack of tension may have allowed the the chain to skip a tooth or two.  though it stops a few degrees short of 1-4 TDC (and thus i can't check it exactly), it appears that the valve timing is still on.

it kinda feels like the chain is doubling back and kinking on the crank sprocket, but i don't believe there should be enough slack even with the tensioner loosened for this to happen. i can't really tell what the hell is going on with the roller assembly, but it's obviously malfunctioning.

anyway, i'm hoping there might be something i'm missing here so that i don't have to pull the head. anyone know what could be going on here?

[Terry in Australia]

G'Day Paul, well even with the tensioner removed, and with the plugs out you should be able to rotate the engine, unless the chain has jumped several teeth and your valve timing is now out, so that when you rotate the engine, the pistons are rising as the valves are opening, making valve to piston contact and stopping the engine from rotating further.

You'll need to remove the engine, take the valve cover off, unbolt the two 6mm screws holding the cam sprocket to the cam, drop the sprocket off and take the cam chain off the sprocket, thus disconnecting the rocker assembly. Now that the valves aren't gonna open (the spring tension will obviously close them) you can (hopefully) rotate the engine with no difficulty.

If you can rotate the engine ok, then rotate it until the #1 and #4 "T" mark lines up on the ignition advancer, then remove the rockers, and rotate the cam until the horizontal marks are in line with the joints on the cam tower and the notch is on top, line the cam sprocket holes up with the threaded holes on the cam sprocket mounting flange, replace the chain, and your cam timing is correct. rotate the engine again (with the rockers still off) so the slack in the chain is at the back of the engine, and install your tensioner and let that spring fly! (don't forget to lock it in this time)

Re-assemble the rockers on the cam, set your clearances, (.002 and .003) re-install the engine, and fire it up! Hopefully you won't have bent a valve, and your free bike will be all good. Have fun! Cheers, Terry.

[paulages]

thanks, terry. the engine is out and on my bench already--like is said, the valve timing appears to be right (the crank/cam rotation stops literally 2-3 degrees short of TDC), so i don't believe the problem to be valve interference. i will probably pull the cam though, just so i can make sure of this. regardless, the cam roller is doing something weird. unless i can figure out why it's doing this (and hopefully is fixable without further disassembly), i'll have to pull the head.

[scondon]

 Engine should spin freely by hand even with the tensioner removed. You're right that there is not enough slack for the chain to double up on itself. If you're seizing up at 2-3 degrees before TDC then it's entirely possible that the chain has skipped a tooth or two. How does the key mark at the end of the cam look at the seize point? Is it pointing "almost" 12 o'clock(2-3 degrees BTDC on 1-4)?

You should be able to remove the two cam sprocket bolts, slide the sprocket off the cam shoulder to get enough slack to take the chain off the sprocket, and jump back a few teeth before pulling everything apart. Might even be able to spin the cam by hand, with the motor position just before the seize point(1 degree) and test whether the valves are being interfered with.

  "it just went clunk and stopped" could be a lot of things. Just thinking out loud here

[754]

Just pull tappet covers and try to turn, and watch for valve hangong up or stuck.

get or make a cylinder light and look thru plug hole for something stuck on the piston..

[paulages]

well, it's definitely not valve interference. i pulled the cam and the problem is the same. i think something happened with the lower roller...the interference is definitely coming from the bottom end or just above. it really feels like it's at the crank sprocket. the malfunctioning chain tensioner can't be a coincidence though. either way, i'll have to pull the cylinders to find out. oh well, guess i'll throw a spare engine in and have this one to play with.

[Jerry Rxman Griffin aka MuthaF'er]

Broken chain?

[754]

Turn upside down and try...

If cam is out, cant you see far enough down the camchaincanyon to see what is holding things up??

[paulages]

the chain's not broken, and no i can't see what's going on, even with the light let in by the tensioner hole. i'll pull the cylinders this afternoon and find out what's going on.

[paulages]

okay... well, the cylinders are removed, and while holding the cam chain up, the crank now rotates freely, telling me that there was definitely some problem with the cam chain and the roller, though the roller appears in perfect condition. don't know what the hell was going on here, but i guess i'm re-ringing and gasketing the goddamn thing.

[scondon]

okay... well, the cylinders are removed, and while holding the cam chain up, the crank now rotates freely, telling me that there was definitely some problem with the cam chain and the roller, though the roller appears in perfect condition. don't know what the hell was going on here, but i guess i'm re-ringing and gasketing the goddamn thing.

    Still trying to wrap my head around how the cam chain roller would be the "source" rather than a symptom of some other problem. If the cam chain was binding somewhere then there should be some indication of it on either the plastic guide/rollers, or the aluminum surface of the tunnel or cases. Hopefully you'll find the "smoking gun" before buttoning everything back up

[Stevien1]

How are things with the primary chain and/or the input shaft to the trans?

[paulages]

How are things with the primary chain and/or the input shaft to the trans?

i haven't split the cases, but with the cylinders removed it spins freely now. i will be nervous putting back together without finding the problem, but it has to be something weird with the roller. the upper wheel just wouldn't stay forward (sprung, i guess). the fact that it was moving back and forth so freely has to be at least related to the problem, but the roller itself looks fine. i think i'll bolt it back up as is, and see if it does at again.

[scondon]

 The rollers actually move back and forth according to the amount of slack/tension in the cam chain. If the chain is pulled taught in the forward direction, and all the slack is taken up at the rear of the chain, it will cause the roller to move back a bit.

   Not saying that it can't be the rollers. You have the engine right in front of you and can determine better than anyone. Just thinking that from your description it still sounds like a symptom of something else. Sprocket teeth on the crank? Bad link in the chain causing it to rise up on a sprocket tooth? How are the rods, any bad slop in any of them?

[HondaMan]

If the camchain is "stretched" enough in miles, and the bottom edge of the slipper tensioner is worn, and the tensioner unit is not engaged (like you've described), the chain can double a link as it enters the lower end of the slipper tensioner, jamming the chain. Unfortunately, the only way I know to test this is to either:

1. Install and "set" the tensioner in the right place to use the slack, then measure the distance it took to fully remove the slack (it should not touch the adjuster bolt with the rising edge of the tensioner's aft shaft end).
2. Pull the cylinders and look at the bottom end of the slipper tensioner.

I once saw a 750 that had been improperly (camchain) set at a California shop and toured at 85+ MPH to Illinois. He came into my shop, complaining that the chain noise was deafening (it really was loud). When I released the camchain tensioner's bolt to set the chain, the whole shaft launched itself past the little 6mm bolt. I disassembled the tensioner from the engine and could see, with a mirror and light, the bare metal on the slipper tensioner. An oil change showed up the metal in the filter, too. Long story shortened: the teardown revealed a fully battered tensioner and chain that measured a full 1.5 links longer than a stock chain! Yet, it still ran 85+ with 2 onboard....

[paulages]

If the camchain is "stretched" enough in miles, and the bottom edge of the slipper tensioner is worn, and the tensioner unit is not engaged (like you've described), the chain can double a link as it enters the lower end of the slipper tensioner, jamming the chain. Unfortunately, the only way I know to test this is to either:

this is exactly what it feels like is happening. mark- i should note that even with the tensioner pushed in as far as i could push it (with a small screwdriver in the end, then tightened down), the top roller was still was doing the "jump" thing. it seems to me that if the chain was doubling over in itself like you are describing, it would pull in the upper wheel, pulling it back (towards the intake side), then return once the chain released, but realistically, it just looked like there was no tension on it at all and it was just flopping around.

with the cylinders pulled, what should i be looking for on the slipper tensioner? it looks identical to one that was functioning when pulled.

[754]

Quick check for really worn chain, just put it on the cam sprocket a half wrap and pullup on chain if it lifts a real lot it is hooped.. also check for roller slop in the cam sprocket teeth.

Did you look over the pistons closely especially if carboned?? even a tiny screw or pebble or anything over a sixteenth of an inch could stop it.

Keep in mind you did remove head and problem has gone away.. with cam out and chain dropped that should have eliminated all chain/roller  causes.

[Terry in Australia]

Well as you've got it down this far Paul, you'd be silly if you didn't crack the cases and replace that cam chain, Z1 enterprises are still selling Tsubaki race chains for under 30 bucks, and you've gotta buy a gasket set anyway?

It'll give you a chance to inspect the crank sprockets to make sure you haven't done any damage there, and to also make sure that there's nothing "spare" sitting in the bottom of the engine?

When I pulled my K6 engine apart recently prior to "Hotrodding" it, I found that the 6mm primary chain tensioner screw was about one thread away from falling into the transmission. This was on an engine that had never been apart in it's life, so was obviously built on a Friday afternoon after the Quality Control guy sneaked off work for a few Saki's with his mates. Cheers, Terry.

[paulages]

Well as you've got it down this far Paul, you'd be silly if you didn't crack the cases and replace that cam chain, Z1 enterprises are still selling Tsubaki race chains for under 30 bucks, and you've gotta buy a gasket set anyway?

It'll give you a chance to inspect the crank sprockets to make sure you haven't done any damage there, and to also make sure that there's nothing "spare" sitting in the bottom of the engine?

When I pulled my K6 engine apart recently prior to "Hotrodding" it, I found that the 6mm primary chain tensioner screw was about one thread away from falling into the transmission. This was on an engine that had never been apart in it's life, so was obviously built on a Friday afternoon after the Quality Control guy sneaked off work for a few Saki's with his mates. Cheers, Terry.

yeah, well rebuilding a 750 engine wasn't really on my plate right now, especially since this one's just for resale, and the 750 i ride is in decent condition. now that i've got it half apart though, i may consider doing the whole shebang. i'm sure i can find something to put it in. i still have at least one spare engine i can through in the bike to sell.

[Terry in Australia]

That's it mate, it's really not a "rebuild" just cracking the cases and replacing the cam chain, but it's gonna give you some piece of mind knowing that the thing is gonna run as good as new, with no "spare parts" floating around in the tranny.

Now I know that it's not in your schedule to paint those gnarly old cases while you've got it apart, (don't ever paint an assembled engine, you might convince yourself that it looks great, but you're not kidding anyone else, we all think it looks crap) but what the heck, a little more elbow grease, and it'll look as nice as it's gonna run!

[paulages]

That's it mate, it's really not a "rebuild" just cracking the cases and replacing the cam chain, but it's gonna give you some piece of mind knowing that the thing is gonna run as good as new, with no "spare parts" floating around in the tranny.

Now I know that it's not in your schedule to paint those gnarly old cases while you've got it apart, (don't ever paint an assembled engine, you might convince yourself that it looks great, but you're not kidding anyone else, we all think it looks crap) but what the heck, a little more elbow grease, and it'll look as nice as it's gonna run!

the problem is, i'm kind of a "well if i'm gonna _____, i might as well _______" kind of guy, so if i split the cases i'll end up doing everything. might skip the overbore and just go for a port job/ hot cam.

[Terry in Australia]

Well a port job and a hot cam are great additions to any engine Paul, but like you, I figure that if you're gonna do it, you may as well shove as much into that engine as you can, the one in the pic has the following tucked inside:

Arias 836cc 12:1 pistons,
RC Rods,
APE Studs,
M3 Racing cam chain tensioner,
Mike Rieck Stage 3 head, and
Megacycle 125/75 cam. 

I'm still playing with the needle positions in those Keihin CR29mm carbs Mike sent me, but I've seen 120 MPH in third gear somewhere just past 11000 RPM, (my tach only reads to 11000, but the needle was still moving south of that point) but I chickened out after the white marker posts on the side of the road started to resemble a white picket fence, ha ha!

Of course the bug has bitten now, and thanks to Mike Kramer I've got a crank on it's way to APE for a weight loss treatment, and when that arrives I'll have to tear the engine down again (or maybe just build another, he he) but after doing two engines this year, it really is a very simple task, and certainly well worth it, an engine should look as good as it goes. Cheers, Terry.

[scondon]

Get one of those KOSO electric tachs,Terry. They go to 15 grand, have a yellow shift light(set anywhere in rpm range) that goes red 500rpm later, and at a press of a button you can check highest rpm attained. Can be used on a assortment of bikes. I la-la-la-la-looooove mine 

[Terry in Australia]

Ooh ooh, where do I get one Sean?

[scondon]

Ooh ooh, where do I get one Sean?

 Canada. Check this thread:

http://forums.sohc4.net/index.php?topic=28280.0

   We now return you to your regular broadcast program, featuring Paulages engine.